Magnetic resonance imaging (MRI) relies on the excitation and relaxation of sub-atomic particles within the structure being imaged, and is an important technology for imaging soft tissue in humans and animals. The inherent relaxation time of the tissue being imaged, however, imposes a limit on the speed at which imaging can take place. Typically, each 2D image (called a “slice”) is built up from several hundred image projection views or lines, which (depending on the required resolution) can each take between 3 to 6 milliseconds to complete. Often, a three dimensional image composed of multiple slices is required. Constructing an MR image is therefore a process that can take several minutes to complete.
In cardiac MRI, the lines in each slice must be acquired over several heartbeats at the same point in the cardiac cycle. Image acquisition is therefore gated using an electrocardiogram (“ECG”) trigger signal, and lines can only be acquired during a short period in each cardiac cycle that the heart is stationary. This short imaging period is referred to as a “segment”, and is generally about 200 ms long in a healthy subject with a resting heart rate of about 70 beats per minutes. Echo spacing refers to the time required to obtain a single view line which, as previously mentioned, is about 3 to 6 milliseconds. Allowing for 30-50 ms for scanner preparation before acquisition, about 25 lines can therefore be obtained per 200 ms imaging segment. For a single high resolution slice of 500 lines, about 20 cardiac cycles of scanning time is needed. Obtaining a high resolution MR image of the heart therefore requires appreciably more time than imaging other soft tissue.
Respiration-induced motion of the heart is a particular problem for high resolution cardiac MR imaging. As the imaging spans several heartbeats, the subject's breathing can cause blurring of the acquired image due to the displacement of the heart within the chest cavity as the subject inhales and exhales.
One technique that is employed to overcome this problem is for a subject to hold his or her breath for an appropriate interval of time. However, patients with heart problems may experience tremendous discomfort and difficulty holding their breath for the appropriate periods of time, such as for 20 heartbeats. Where more than one breath-hold is required to complete a slice the position at which the patient holds their breath is rarely identical.
Techniques that allow normal or quiet breathing generally involve the use of navigator signals. A navigator signal is a scanning sequence made by the MR scanner prior to each segment, which involves the excitation of a narrow line across a structure affected by the respiration. Typically, the navigator sequence is made across the right hemi diaphragm. The high contrast been the lung and the liver above and below the diaphragm allows easy detection of the position of the diaphragm, and the position of the heart can be accurately estimated based on the position of the diaphragm.
One technique for compensating for respiratory motion using navigator signals is called the “accept-reject algorithm”, in which segments are only taken when the diaphragm is in a certain position, usually within a 5 mm window of acceptance. The drawback of this technique is that it further reduces the time available in which segments can be acquired, leading to very long imaging times.
Another technique, called the “gate and follow” approach, uses the navigator position immediately prior to the imaging segment to correct the positions of the lines made throughout the segment. However, in cases where the subject's breathing causes respiratory motion of the heart during the segment, the navigator data becomes out-dated as the segment duration increases, resulting in images that are not sharp enough. The gate and follow approach is generally not used for this reason.